The invention relates to a system and method for controlling an internal combustion engine coupled to an emission control device.
Engine and vehicle fuel efficiency can be improved by lean burn internal combustion engines. To reduce emissions, these lean burn engines are coupled to emission control devices known as three-way catalytic converters optimized to reduce CO, HC, and NOx. When operating at air-fuel ratio mixtures lean of stoichiometry, another three way catalyst known as a NOx trap or catalyst is typically coupled downstream of the three-way catalytic converter, where the NOx trap is optimized to further reduce NOx. The NOx trap typically stores NOx when the engine operates lean and releases NOx to be reduced when the engine operates rich or stoichiometry.
One method for controlling air-fuel ratio to release, or purge, stored NOx operates the engine rich until an air-fuel sensor downstream of the NOx trap indicates a rich air-fuel ratio. In other words, while the air-fuel ratio entering the NOx trap is rich, the output air-fuel ratio exiting the NOx trap will be near stoichiometry until a majority of the stored NOx is released. When the air-fuel ratio downstream becomes rich, there is little stored NOx and thus hydrocarbons are not used to reduce NOx and exit. Stated another way, the NOx trap is purged of stored NOx. Then, the engine air-fuel ratio can again become lean and the NOx trap can again store NOx. Such a system is described in EP 733786.
The inventors herein have recognized a disadvantage of the above approach. In particular, when the air-fuel sensor is placed downstream of the NOx trap, there is always extra fuel used. In other words, since there is a delay from when fuel is injected until it reaches the air-fuel sensor, there will always be a certain amount of rich exhaust in the exhaust system when a purge is ended. All of the fuel in this bit of rich exhaust is excess and degrades fuel economy.
As an attempt to solve the above disadvantages, another approach is to place the air-fuel sensor somewhere in the NOx trap. In other words, the air-fuel sensor may be placed at a location two-thirds from the front of the NOx trap. In this way, there is still some catalyst material after the air-fuel sensor to use the excess fuel in the rich exhaust.
The inventors herein have recognized a further disadvantage with the above approach. In particular, to obtain optimum performance, the sensor location is dependent on exhaust mass flow. Stated another way, at high exhaust mass flows, the sensor should be located closer to the front of the catalyst since a greater amount of fuel will be stored in the exhaust. Similarly, at low exhaust mass flows, the sensor should be located closer to the rear of the catalyst. Since only a single location is practical, performance is degraded.
An object of the invention claimed herein is to provide a method for controlling an engine during emission control device purging.
The above object is achieved, and disadvantages of prior approaches overcome, by claim 1.
By using a less rich value to complete purging of the emission control device, only a small amount of fuel is stored in the exhaust system when a purge completion signal is obtained. Thus, minimal emissions are produced during purging. Also, total purge time is minimized since most purge fuel is supplied at the richer air-fuel ratio.
An advantage of the above aspect of the present invention is that over purging is minimized.
Another advantage of the above aspect of the present invention is that fuel economy is optimized while excess rich emissions are also minimized.
In another aspect of the present invention, the disadvantages of prior approaches are overcome by a method for controlling an internal combustion engine coupled to an emission control device with an exhaust sensor coupled downstream of the emission control device, the method comprising: operating the engine at a lean air-fuel ratio during a first interval, operating the engine at a first rich air-fuel ratio during a second interval following said first interval, and operating the engine at a second rich air-fuel ratio during a third interval following said second interval, wherein a duration of said second interval is based on a parameter indicative of a fuel quantity used during previously performed second and third intervals.
By adaptively adjusting the first rich interval, it is possible to account for catalyst aging, while minimizing the rich operating time.
Other objects, features and advantages of the present invention will be readily appreciated by the reader of this specification.